Small internal combustion engines, utilize carburetors to deliver a mixture of fuel-and-air to an intake manifold of the engine. The carburetor body typically carries a fuel-and-air mixing passage which is restricted by a venturi between an inlet or upstream region and a downstream region of the passage. Clean air at substantially atmospheric pressure typically flows from an air filter and through the upstream region where the air flow rate is controlled by a choke valve disposed pivotally in the upstream region. Clean air flowing past the choke valve increases in velocity as it enters the venturi region thus creating a vacuum or sub-atmospheric condition at the venturi region which causes liquid fuel to flow from a fuel chamber at atmospheric pressure, through a primary fuel feed passage and into the venturi region from a fuel orifice or nozzle disposed at the radially inner most location of the venturi. The entering fuel mixes with the incoming clean air and flows as a mixture through the downstream region of the fuel-and-air mixing passage. The volumetric flow rate of the fuel-and-air mixture entering the engine intake manifold is controlled generally by a throttle valve disposed pivotally in the downstream region of passage. When the engine is idling or at low power conditions, the throttle valve is substantially closed thus limiting the rate of air flowing through the mixing passage which reduces the vacuum at the venturi region and in-turn reduces the rate of fuel flowing through the primary fuel feed passage.
During design and/or manufacturing of the carburetor, the primary fuel feed passage (i.e. diameter and/or flow restrictor size) must be properly sized or calibrated in conjunction with changing dynamics within the mixing passage to produce a smooth running engine throughout its operating power range, and which meets government regulatory emission requirements. For instance, if the engine runs too rich at idle, excessive carbon monoxide, CO, is expelled through the engine exhaust, yet if the engine runs too lean at wide open throttle or higher power demand conditions excessive NOx emissions are produced. Unfortunately, via calibration of the primary fuel feed passage alone, reducing NOx emissions at high power operating conditions by increasing fuel flow increases CO emissions at lower power conditions, and reducing CO emissions at low power conditions by reducing fuel flow increases NOx emissions during high power conditions. Moreover, such calibration restraints have the effect of reducing the overall fuel efficient power range of an operating engine in order to meet government regulatory emission requirements.